Magneto-optical discs have a magneto-optical recording layer on a grooved substrate. On recording and reading, tracking control signals and focus control signals are detected in terms of the magnitude of reflected light from the disc surface.
In pre-formatted magneto-optical discs, some bits of information are previously recorded as pits in a lead-in or lead-out area in addition to the groove, the pre-recorded information including information for allowing the hardware to operate the disc under optimum conditions and discrete value information in the form of pulse signals of a predetermined period. Therefore, the pre-formatted magneto-optical disc includes both a region where pits are formed and a region where a groove is formed.
In these magneto-optical discs, since such operation as tracking servo, seek servo and pit information retrieval is carried out by means of an optical system using the same laser light in the pit and groove forming regions, the sizes of pits and groove must be optimized for the optical system in order to prevent occurrence of errors in the servo systems and pit information retrieval.